The exhaust jet of a gas turbine engine remains a significant noise source, particularly at high power conditions, despite the use of high by-pass ratios in modern aircraft engines which has brought several significant benefits particularly in fuel efficiency and noise. Jet noise in a turbofan engine is caused by the interaction of the air streams within the engine and also with the surrounding air. The jet noise comprises turbulent mixing noise, which is at least in part caused by: a) mixing of the core and bypass flow streams; and b) mixing of the mixed stream with the ambient air creating a shear layer. The turbulent shear flow mixing includes two types of noise, the first caused by fine scale turbulence and the second caused by large scale eddies.
The prior art proposes reducing noise levels by modifying the exhaust nozzle area, such as by including bumps or wave-like surfaces or by adding chevron shaped tabs on the exhaust nozzle.